A traditional method of generating constant-envelope frequency shift keyed, FSK, data modulation employs a phase-locked loop, PLL, to maintain the frequency of an associated voltage controlled oscillator, VCO, with its stability derived from the PLL's reference frequency. The PLL's frequency reference typically consists of a crystal oscillator. Data is applied to the VCO which then provides the FSK modulation. This method has the capability of sustaining fairly high levels of frequency deviation as compared to the older technique of direct frequency modulation of a crystal oscillator. This capability is important in moderate to high data rate systems where the frequency deviation is typically an appreciable fraction --e.g., 25%-- of the data rate. In combination with a frequency offset transceiver architecture, the PLL is capable of accurate and stable operation over a wide range of operating frequencies.
A problem that commonly occurs with PLL modulators in data systems relates to the loop bandwidth. While data that has frequency content substantially above the loop bandwidth is able to modulate the VCO satisfactorily, if there is frequency content that falls inside the loop bandwidth--such as that which occurs with long strings of zeros or ones--destructive distortion of the data stream will take place which will render the data unrecoverable by an intended receiver. This issue has been addressed in various ways. Some methods employ a dual-port modulation scheme. This approach extends the PLL response towards direct current, DC, but never quite reaches it because of dynamic range limitations of some element of the loop. Other methods use complex schemes that read incoming data and reprogram loop dividers to achieve a DC response. Still other methods are reconciled to the high-pass response of the PLL frequency modulator and use data "whiteners" that randomize data to deal with long strings of zeros or ones, but still have to contend with vexatious patterns that frequently confound such attempts.
There exists, therefore, a need for a method, device, and radio for compensating for modulator frequency drift while allowing for data transmission, wherein the method and device is substantially less complex than prior art.